Recovery of aromatics by extraction with solvent mixture of n-methyl pyrrolidone and monoethanolamine



Dea 10, 1968 KARL-HEINZ EISENLOHR ETAL 3,415,741

RECOVERY OF AROMATICS BY EXTRACTION WITH SOLVENT MIXTURE OF NMETHYL PYRROLIDONE AND MONOETHANOLAMINE Filed Dec. 20, 1967 5 Sheets-Sheet l Im mp FCPA/EVS DCC 10, 1968 KARL-HEINZ EisENLoHR ETAL 3,415,741

.RECOVERY OF AROMATICS BY EXTRACTION WITH SOLVENT MIXTURE OF N-METHYL PYRROLIDONE AND MONOETHANOLAMINE Filed Dec. 20, 1967 3 Sheets-Sheet f am Mag Dec. 10, 1968 KARL-Hamz ElsENLoHR l-:T AL 15,741

OMAT ACTION WIT RECOVERY OF AR ICS BY EXTR H SOLVENT URE OF' NMETHYL PYRROLIDONE AND MONOETHANOLAMINE 3 Sheets-Sheet 5 Filed Dec. 20, 1967 XvXA Zyg- AVAVAVAVA VVVVV /WWWX Nvvw/WA /Wzvmexs United States Patent O ABSTRACT F THE DISCLOSURE Aromatic hydrocarbons are extracted with solvent from hydrocarbon mixtures containing both aromatic and nonaromatic hydrocarbon components using a mixture of N- methyl pyrrolidone and monoethanolamine as the solvent. The extractions are conducted by liquid-liquid extraction.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation in part of copending application Ser. No. 490,818, entitled Recovery of Aromatics by Extraction or Extractive Distillation with Solvent Mixtures, and tiled Sept. 2.8, 1965, now Patent 3,366,568, in the names of Karl-Heinz Eisenlohr and Eckart Mller.

BACKGROUND OF THE INVENTION Field of the invention The present invention provides an improved process for the recovery of aromatic hydrocarbons, especially benzene, toluene and xylene, from hydrocarbon mixtures containing the same, by extraction with water free solvent mixtures selective for aromatics which as a W-hole boil over the boiling point range of the aromatics to be recovered. The extraction with the solvent mixtures concerned is by liquid-liquid extraction.

Description of the prior art The recovery of aromatic hydrocarbons from hydrocarbon mixtures by liquid-liquid extraction with selective solvents has been practiced on a commercial scale for several decades. However, from the fact that again and again new solvents are suggested and patented for this purpose, it can be concluded that the ideal solvent has as yet not been found.

In addition to pure solvents, in many instances organic solvent lmixtures have been proposed as the selective solvents. Furthermore, widely Aspread practice is to add certain quantities of water to the solvent. The addition of water offers the following advantages:

(1) The selectivity of the solvent is increased.

(2) The separation, especially of the higher aromatics, from the extract obtained is easily accomplished even with a small diierence between the boiling point of the solvent and the aromatics because the aromatics distill azeotropically with the water at temperatures under 100 C.

(3) The sump temperature in the distillation column employed for separation of the aromatics from the solvent extract is lowered 4so that not only are losses engendered by thermal decompositions reduced but also the heat energy required for heating the solvent to the sump temperature is lowered.

(4) With regard to liquid-liquid extractions a further advantage is that the field of the existence of two phases ICC is increased, so that larger quantities of aromatics may be contained in the feed stock without leaving the twophase lield.

On the other hand, the following disadvantages of the addition of water must be taken into consideration:

(l) The solvent capacity for aromatics is reduced.

(2) The water is vaporized azeotropically with the aromatics to be separated from the solvent which causes substantial increase in the heat energy required.

(3) In many instances the water causes undesired chemical reactions, especially hydrolysis.

A number of non-aqueous solvent mixtures have also become known, such as, for example, mixtures of SO2, ethylene glycol and formamide or mixtures which consist of a primary solvent which contains glycol derivatives and a secondary solvent such as methanol, ethanol and acetone.

These solvent mixtures, however, have the disadvantage that a portion thereof has a lower boiling point than the aromatics to be recovered whereas the other portion has a higher boiling point. The separation of the aromatics from the solvent therefor requires substantial plant investment as well as high operation costs which in general are not compensated for by the advantages of the solvent combinations.

Further proposals for solvents include, for example, (l) the combination of various glycol derivatives, (2) mixtures of two solvents of which the first contains 1 or 2 hydroxyl groups and the second contains 2 or more hydroxyl groups, (3) ethylene carbonate with additions of glycerol, ethylene glycol, pentaerytbritol, formamide, formic acid, ethanol amines, monochlorohydrin, acetamide, resorcinol or hydroquinone as diluent, (4) alkane dinitriles, dimethyl hydantoin, N-alkylpyrrolidones, -butyrolactone, cyan ethers of diethylene glycol, trior tetraethylene glycols or any desired mixtures of these solvents which are selective for aromatics or oletns.

However, without exception, no advantages over the use of a single solvent have Ibeen proved for these and many other proposals for the use of mixtures of various selective solvents boiling above the aromatics to be recovered for the liquid-liquid extraction of aromatics, nor have any indications been given as to what quantity and in what proportions the various components of the mixture should be used in order to attain advantages over the use of single solvents.

The state of the art concerning the possibilities of use of solvent combinations for the liquid-liquid extraction of aromatics can be summarized in that the effect of the addition of water to a solvent is known qualitatively, and that something is known concerning the properties of various combinations of glycol derivatives. However, from which points of view, solvents may otherwise be combined in order that they give good properties for liquid-liquid extraction of aromatics has neither been described in scientific literature nor in the patent literature.

SUMMARY OF THE INVENTION The object of the present invention is to provide a process for the recovery of aromatic hydrocarbons from hydrocarbon mixtures containing the same by means 0f a liquid-liquid extraction process.

The essence of the present invention lies in the Iuse, as the liquid extracting medium, of a water tree solvent mixture which boils above the boiling point range of the aromatics to be recovered and which consists of a mixture of N-methyl pyrrolidone and monoethanol amine.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: FIG. 1 is a diagram showing the maximum quantity of 3 benzene in the heavy phase (solvent phase) with respect to the composition of the solvent mixture N-methyl pyrrolidone (hereinafter referred to as NMP) and monoethanolamine;

FIG. 2 is a diagram showing the dependency of the selectivity of the composition of solvent mixture NMP and monoethanolamine;

FIG. 3 is a diagram in which the selectivity is plotted against the partition-coecient in dependence on the composition of solvent mixture NMP and monoethanolamine;

FIG. 4 is a phase diagram for the 3 component system NMP-monoethanolamine-benzene;

FIG. 5 is a diagram showing the dependency of the viscosity at 50 C. of solvent mixture NMP and monoethanolamine on its composition; and

FIG. 6 schematically shows an apparatus for carrying out a liquid-liquid extraction according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT According to the invention, it was unexpectedly found that despite the large number of solvents known for these purposes and despite their very different types of chemical structure, certain rules exist, the application of which renders it possible to select from the boundless large number of possible combinations, those which can be expected to have optimal properties.

Above all the following physical properties are desired in a selective solvent for the recovery of aromatics by liquid-liquid extraction.

(l) High selectivity.

(2) High capacity.

(3) High aromatics range, that is, the formation of a two-phase system even at high aromatic concentrations.

(In the following, the highest aromatics concentration in the 3 substance system benzene-nheptane-solvent at which a two-phase system is still present and above which separation into two phases no longer occurs is indicated as the aromatics range.)

(4) Low solubility of the solvent in the hydrocarbon phase.

(5) Low viscosity.

(6) High density.

(7) Boiling point above the boiling point range of the aromatics to be recovered.

(8) Thermal and chemical stability at boiling point.

(9) Melting point below ambient temperature.

It was ascertained that these various physical properties do not occur with complete irregularity in the individual solvents but rather that certain conformities to rules can be observed which are of special significance for the selection of a favorable solvent combination.

Of the properties given above, the third, namely, the aromatics range is especially suited as the classifying principle for dividing the solvents into certain groups. If the solvents are arranged according to increasing aromatics range they can be divided into three groups:

(A) Solvents with low aromatics range.

(B) Solvents with high aromatics range and (C) Solvents which do not have unlimited miscibility with aromatics.

A low aromatics range is one of less than 50% by weight and preferably less than by weight is the maximum of the two-phase region.

The solvents of the rst group A are characterized by their high capacity, low selectivity and high solubility of the solvent in the hydrocarbon phase. In view of their low selectivity the solvents of this group normally are only employed in combination with water whereby the abovementioned disadvantages must be accepted.

It has now been found that most of the solvents of this group possess two other important and advantageous properties, namely, low viscosity and good thermal stability at the boiling point. Compounds with heterocyclic ring systems, such as, morpholine, N-methyl pyrrolidone, furfural, keto dioxane, and also dimethyl formamide, aniline, ethylene diamine, nitromethane, ethylene glycol monomethyl ether, dicthylene glycol mono-methyl ether, and others, all of which have an aromatics range below 35%, and dicthylene, triamine, triethylene tetramine, tetraethylene pentamine, butyrolactone, and dimethyl sulfoxide with aromatics ranges between 35 and 5 0% especially are illustrative of the group of solvents with low aromatics range.

The solvents of group B with a high aromatic range, that is, solvents with an aromatic range of at least 50% in the maximum of the two-phase system belong to the second group. They are therefore near the solubility limit with aromatics, for example, so that they are miscible in all proportions with benzene but not with xylene.

There are solvents of this group which have very high selectivity with good capacity, the latter, nevertheless, being lower than that of the solvents of group A. As a consequence, solvents of group B would be the ideal solvents for the extraction of aromatics insofar as only the selectivity and capacity is taken into consideration, but remarkably they without exception possess other physical properties of such unfavorable values that they must be considered substantial disadvantages. Above all, representatives of this group all have the disadvantages that they are no longer stable at their boiling point and have a high viscosity. In addition, their melting points in some instances are above normal temperature.

Compounds with double bonded oxygen and dinitriles, such as, for example, sulfolane, ethylene and propylene carbonate, oxy, thioand imino-dipropionitrile, monomethyl formamide and tetraethylene glycol especially are illustrative of group B solvents.

The solvents of the third group C have a miscibility gap with aromatics, that means, they are not miscible in all proportions with aromatic hydrocarbons. Representatives of this group which dissolve less than 25% of benzene at 20 C. are preferably employed. Their capacity is less than that of the solvents of `both other groups. Their selectivity is better than that of solvents of the first group. Almost all have a high viscosity. Thermal stability at the `boiling point is sometimes given. This group mostly contains chain compounds with one or more hydroxyl groups, often also compounds with amino groups, such as, for example, ethylene glycol, dicthylene glycol, propylene glycol, glycerol, mono, diand triethanol amine, 1.4-cyclohexane dimethanol, diglycol amine, formamide, malondinitrile, hydrazine and others.

According to the invention it was found that solvent mixtures could be obtained which have substantially better properties than could be expected from the rule of mixtures by mixing a solvent having a low aromatic range belonging to the tirst group (A) with a solvent having a miscibility gap with aromatics and belonging to the third group (C). In each instance the selectivity is better than could be expected from the rule of mixtures. The ability to take up aromatics with solvents of group A is limited `by their aromatics range, and with solvents of group C by their low solvent capacity for aromatics. The combination of a solvent of group A with a solvent of group C results in a mixture having an ability to take up aromatics which is always better than that of each component and which often exceeds that of the component with the best take up ability by 50% and more.

An undesirable characteristic of solvents of group C is their high viscosity. In this respect, an unexpected advantage also is found in the combinations according to the invention as the viscosity of the solvent mixture is always less than could be expected from the rule of mixtures.

In addition, it was found that the best mixing solution for the various solvent mixtures can quickly be found according to a very definite method:

One investigates the three component system, solvent A-solvent C-benzene, determines the solubility limits and then the critical point (plait point), that is, the point where at the solubility limit the volumes of the light and heavier phases are equal. The composition at this point is the most favorable composition for the liquidliquid extraction of aromatics.

The critical point for a particular composition of a solvent A and solvent C, with the prerequisite that solvent A and benzene and solvent A and solvent C are completely miscible and that solvent C and benzene have a miscibility gap, is determined as follows:

Equal parts by volume of benzene and solvent C are placed in `a vessel. Two phases are formed, one of which predominantly `contains benzene-generally the lighter phase-in the following designated as the HC-phase (hydrocarbon phase) and one which predominantly contains solvent C-generally the heavy phasedesignated in the following as the S-phase (solvent phase). Then small increments of solvents A are added while observing how both phases behave or change with respect to each other. When a certain quantity of solvent A has been added the phase separation line will suddenly disappear and only one phase will be present. If by chance the volumes of the phases have been equal just before disappearance of the phase separation line the critical point would already have been determined. The critical point lies between the composition where two phases are last observed and the composition at which the phase separation line disappears. Usually, however, the volumes of the phases will be different and shortly before disappearance of the phase separation line the phase present in the larger quantity will be a multiple of Athe other phase. The procedure which follows under such circumstances depends upon which of the two phases was the larger just before disappearance of the phase separation line. If the HC-phase is Ithe smaller one, small increments of benzene are added to the mixture until the mixture becomes cloudy and again separates into two phases. If at this point the HC-phase is still the smaller one, then a Amixture of equal parts of benzene and solvent A are added in small increments until the phase separation line again disappears so that only one phase is present. Thereafter this procedure is repeated, that is, so much benzene is added until clouding and phase separation occurs and then so much of an equal mixture of benzene and solvent A is `added until the phase separation line disappears, at some point the condi-t-ion will occur in which the S-phase rather than the HC-phase is the smaller. At this point the critical point has -already been exceeded and one can usually determine its position by interpolating between the last and second last measuring point. However, if this is not `believed sufficiently accurate, because the quantities added were selected too large, several mixtures are prepared which are near the critical point and which still just separate into two phases and a mixture of equal quantities of benzene and solvent A is added thereto in small increments. The mixture in which the two phases are of equal volume just rbefore the phase separation line disappears corresponds to the composition of the critical point.

If, on -the other hand, the S-phase is smaller just before disappearance of the phase separation line during the rst addition of solvent A the procedure is analogous but the benzene added to cause phase separation is replaced by solvent C and the mixture f equal parts of benzene and solvent A added to cause disappearance of the phase separation line is replaced by a mixture of equal parts of solvent C and solvent A and the procedure repeated until the S-phase just becomes the larger phase and the critical point interpolated or determined analogously to the above procedures.

When the feed stock from which the aromatics are to be recovered is one in which the non-aromatics predornnantly are parafns or if a parainic antisolvent is used the quantity of solvent C in the mixture can be to -less than previously dened. If, on the other hand, the nonaromatics in the feed stockcontains large quantities of olefins and/or naphthenes it often is expedient to increase the quantity of solvent C in the mixture about 5 to 10%.

Preferred mixtures of solvents of the group A with solvents of group C are as follows:

*M2-amino ethoxy) ethanol or oxethoxyethylamine.

These relationships are more fully explained in the following with reference to FIGS. 1-6 with the mixture of N-methyl pyrrolidone (abbreviated as NMP) and monoethanolamine :as example. As can be seen from FIG. l the ability to take up aromatics of a mixture of a solvent of group A (NMP) and a solvent of group C (monoethanolamine) is better than that which corresponds to the rule of mixtures and is represented by the broken line A-C. In general, the ability of a solvent mixture of group A with a solvent of group C to take up aromatics is greater than that of each solvent individually. In the illustration given the maximum take up is achieved with a mixture of 67% of monoethanolamine and 33% of NMP and amounts to 53 weight percent of benzene in the heavy phase. The dependency of the selectively upon the solvent mixture composition, once for 10% benzene and once for 40% benzene in the light phase, is shown in FIG. 2. Both curves run above the tie line between the points 0f the pure solvent compound.

The manner in which the optimum between selectivity and capacity can be determined is shown in FIG. 3. In such ligure the capacities (partition coefficients) and the selectivities for different mixing ratios of NMP and monoethanolamine are respectively plotted as abscissa and ordinates. All data are based on a 3 component mixt-ure of solvent-benzene-heptane at a ratio of 70:20:10 parts by weight in order to have comparable results. A logarithmic scale Was chosen since the relative change of the two characteristics is more important than their obsolute change. It can be seen that the points of the individual mixtures lie above and to the right from the line A-C and therefore towards values of higher capacity and higher selectivity. It furthermore can be seen that the range which extends furthest up and right and therefore the optimal economic range lies between 60 and 70% monoethanolamine.

This value of about 65% monoethanolamine is also found when the three component diagram of both solvents with benzene is investigated and the critical point is determined as described above. FIG. 4 gives the diagram for the system NMP-monoethanolamine-benzene. At the critical point P the ratio of monoethanolamine to NMP is 67:33.

It also can be seen from FIG. 5 that the viscosity at 50 C. of a mixture of NMP and monoethanolamine, :above all in the middle range, is significantly lower than was to be expected from the rule of mixtures.

The present invention is generally suited for the recovery of aromatics from hydrocarbon mixtures containing aromatics by liquid-liquid extraction and also for the recovery of extracts in which the concentration of .aromatics in the hydrocarbon extracts has only been increased over that in the starting mixture. An especial advantage of the process according to the invention is that it also is adapted for the recovery of high purity aromatics, which in recent times have assumed considerable signicance as starting materials for chemical syntheses.

The process according to the invention renders it possible to recover aromatics whose non-aromatics content is below 0.1% and even under 0.01% without difficulty. Even higher purity requirements such as absence of nonaromatics in quantities capable of chromatographic detection can be achieved according to the invention.

The invention is further illustrated by the following example and FIG. 6 which diagrammatically shows the apparatus employed in such example.

Example 2000 kg./h. of a catalytical reformate of the following composition Wt. percent B (benzene) l T (toluene) 39 X (xylene) 4 Nonaromatic hydrocarbons 47 Wt. percent B 8.1 T 11.6 X 0.7 Nonaromatic hydrocarbons 5.6 Solvent 73.

were withdrawn from the bottom of extractor 2 and supplied to the preliminary distillation column 4 over conduit 3. Column 4 had 40 actual trays and was operated with a reflux ratio of 0.511. 800 kg./h. of the head product which consisted of Wt. percent B 45 T 5 Nonaromatic hydrocarbons 48 Solvent 2 was recycled to extractor 2 as reflux over line 5. This head product contained all of the non-aromatics originally contained in the extract. 1000 kg./h. of a raffinate of the composition Wt. percent T 3 X 3 Nonaromatie hydrocarbons 94 were withdrawn from the extractor through line 6.

6000 kg./ h. of a non-aromatics free extract were withdrawn from the sump of column 4 through line 7 and supplied to solvent stripper 8 which contained 20 actual trays and was operated with a reflux ratio of 1:1. 1000 kg./h. of pure aromatics of the composition Wt. percent were taken off overhead through conduit 9. The sump product (5000 kg./h.), which was practically pure solvent mixture, Was recycled to extractor 2 through line 10.

The aromatics mixture taken off overhead from stripper 8 was distilled in a known manner to separate it into its components. The benzene recovered had a melting point of 550 C., the toluene recovered had a refractive index nD2=l.4967 and no non-aromatics were discernible gas chromatographically in the xylene fraction.

We claim:

1. A process for separating a hydrocarbon mixture containing aromatic and non-aromatic hydrocarbons into fractions of different degrees of aromaticity by solvent extraction with a solvent mixture and recovering a more concentrated aromatics fraction from said mixture which comprises contacting the mixture with a water free solvent mixture which as a whole boils above the aromatics to be recovered, said solvent mixture consisting of solvent (A), N-methyl-pyrrolidone, and solvent (C), monoethanolamine, the ratio of the quantity of said solvent A to be quantity of said solvent C in said sol-vent mixture being from X: Y to X :YilO weight percent, the ratio X :Y being the ratio of solvents A and C at the critical point in the three component system solvent A, solvent C and benzene, said solvent mixture being a mixture of 57 to 77% of monoethanolamine and 43 to 23 weight percent of N-methyl-pyrrolidone, effecting phase separation of the phases thus formed, an extract phase in which the hydrocarbons extracted from said mixture has a higher aromatic concentration than said mixture and a rainate phase less aromatic than said mixture and distilling an arom'atics rich hydrocarbon fraction from the solvent mixture in said extract phase.

No references cited.

DELBERT E. GANTZ, Primary Examiner.

H. LEVlNE, Assistant Examiner.

U.S. Cl. X.R. 260-674 

